Eating large quantities of food in the middle of the night. Sometimes the person is sleepwalking and doesn't even know they are doing it. But other times it is a voracious craving probably brought on by austere dieting.

A person who does the act is a werepig.

I heard you in the kitchen last night. Were you werepigging again?

I woke up this morning and the chips were open and the salsa was on the counter. You need to clean up your werepigging mess!